The powder metallurgy industry has developed metal-based powder compositions, generally iron-based powders, that can be processed into integral metal parts having various shapes and sizes for uses in various industries, including the automotive and electronics industries. One processing technique for producing the parts from the base powders is to charge the powder into a die cavity and compact the powder under high pressures. The resultant green compact is then removed from the die cavity and sintered to form the final part.
To avoid excessive wear on the die cavity, lubricants are commonly used during the compaction process. Lubricants can be generally classified into two groups: internal (dry) lubricants and external (spray) lubricants. The internal lubricants are admixed with the metal-based powder composition, and the external lubricants are sprayed onto the die cavity prior to compaction. Lubricants are used to reduce internal friction between particles during compaction, to permit easier ejection of the compact from the die cavity, to reduce die wear, and/or to allow more uniform compaction of the metal powder blend. Common lubricants include solids such as metallic stearates or synthetic waxes.
As will be recognized, most known internal lubricants reduce the green strength of the compact. It is believed that during compaction the internal lubricant is exuded between iron and/or alloying metal particles such that it fills the pore volume between the particles and interferes with particle-to-particle bonding. Indeed, some shapes cannot be pressed using known internal lubricants. Tall, thin-walled bushings, for example, require large amounts of internal lubricant to overcome die wall friction and reduce the required ejection force. Such levels of internal lubricant, however, typically reduce green strength to the point that the resulting compacts crumble upon ejection. Also, internal lubricants such as zinc stearate often adversely affect powder flow rate and apparent density, as well as green density of the compact, particularly at higher compaction pressures. Moreover, excessive amounts of internal lubricants can lead to compacts having poor dimensional integrity, and volatized lubricant can form soot on the heating elements of the sintering furnace. To avoid these problems, it is known to use an external spray lubricant rather than an internal lubricant. However, the use of external lubricants increases the compaction cycle time and leads to less uniform compaction.
Accordingly, there exists a need in the art for metallurgical powder compositions that can be readily compacted to strong green parts that are easily ejected from die cavities without the need for an external lubricant. One solution to this problem is to employ powder compositions such as those set forth in U.S. Pat. No. 5,290,336 to Luk, assigned to Hoeganaes Corporation. The U.S. Pat. No. 5,290,336 patent discloses the use of a polyether with a dibasic organic acid to both increase green strength properties and to act as a binding agent. These compositions are preferably prepared using a solvent for the dibasic organic acid, and such solvent preparation methods can increase the costs of manufacture. The compositions of the present invention are preferable to those disclosed in the U.S. Pat. No. 5,290,336 patent in that the dibasic organic acid is not required, and there is no need for a solvent-based blending process.